Girders are commonly used in the construction of structures, such as bridges, to support vertical loads. The girders used are mainly of two types, i.e. girders which are entirely of steel and girders which are constructed entirely of reinforced concrete. In cross-section, the girders have a vertical central web portion and horizontal flanges at the opposite ends of the web portion. The concrete girders have the disadvantage that they require costly form systems to manufacture and prestressing by strands is usually necessary, which is expensive. Although the product is relatively cheap, it is heavy, and this results in high transportation costs, as well as high erection costs. Concrete beams can generally not be cantilevered. Steel girders, on the other hand, have the disadvantage that they are formed from an expensive raw material, otherwise, they have the advantage of being light, easy to erect and they can be cantilevered.
It is an object of the present invention to provide a girder which is a composite structure comprising both reinforced concrete and steel, thereby to minimize the disadvantages of the pure reinforced concrete and pure steel girders and yet to obtain the advantages of these two types of girders in a single structure.